It is well known that the diabetic is particularly susceptible to infection. Current reports indicate that the immune function in the diabetic is defective. The thymus, a key organ in the immune system, is not considered to be a major target organ for insulin, but it is now well documented that thymocytes and proliferating lymphocytes have specific binding sites for insulin. The metabolic effects of insulin on these cells, however, have not been well characterized. In previous work, the applicant has shown that insulin in vitro displays a stimulatory effect on the glucose metabolism of rat thymocytes. It is not known, however, which cell type in a heterogeneous population of cells in the thymus binds and responds to insulin. Experiments are planned to determine which class of cells in the thymus is insulin sensitive. Binding of insulin-immunofluorescent antibodies to frozen thymus sections will be examined. Thymocytes will be fractionated on nylon-wool columns, by density gradient centrifugation, and affinity chromatography on Sepharose-insulin columns. Subpopulations of thymocytes will be characterized for insulin binding, the effect of insulin on glucose metabolism, and activity of terminal deoxynucleotidyl transferase. The effect of insulin will be further characterized by determining the conditions for an optimal insulin response. Parameters that will be examined are ionic milieu, pH, temperature, buffer, etc. The role of insulin in fostering lymphatic tissue growth and maintenance of thymus integrity will be examined by studying the effect of insulin in the diabetic rat. Insulin binding to, and the effect of insulin on glucose metabolism of thymocytes from diabetic rats will be examined. Thymus tissue will be involuted with glucocorticoid administration and its growth and regeneration monitored in normal and diabetic rats by measuring the incorporation of 3H-deoxynucleotides into thymic DNA. These studies will lead to a better understanding of the interaction of insulin with lymphatic tissue, and will shed light on the immunological defects observed in the diabetic individual.